This invention relates to a method and apparatus for moderating or controlling the combustion of fuel modules charged into a combustible environment. More specifically, the present invention relates to a method and apparatus for controlling the rate at which fuel modules are charged into the riser duct of a preheater or precalciner kiln.
This invention relates to a method and apparatus for achieving environmentally sound disposal of combustible solid waste in an operating preheater or precalciner cement kiln. In the widely used commercial process for the production of cement clinker, cement raw materials are calcined and xe2x80x9cclinkeredxe2x80x9d by passing finely divided raw mineral materials through a rotating inclined rotary kiln vessel or kiln cylinder. The requisite temperatures for processing the mineral material are achieved by burning fuel such as gas, fuel oil, powdered coal and the like at the lower end of the kiln with the kiln gases moving countercurrent to the mineral materials moving through the rotating kiln cylinder.
Preheater and precalciner cement kilns are well known in the art and a description of their operation may be found in U.S. Pat. No. 5,816,795 to Hansen et al (xe2x80x9cthe ""795 patentxe2x80x9d) that is incorporated by reference herein. Preheater or precalciner kilns have, in addition to an inclined rotating kiln vessel fired at its lower end, a stationary heat transfer portion at its upper end (typically including multistage cyclones) for preheating and/or precalcining the mineral material before it is introduced into the upper end of the rotating kiln vessel. Because the mineral material is preheated or precalcined before entering the rotating kiln vessel, the length of the rotating kiln vessel can be much shorter than the rotary vessel in conventional long kilns. The present invention provides a method and apparatus for controlled environmentally sound, highly efficient burning of solid combustible wastes as supplemental fuel in the stationary heat transfer portion of preheater or precalciner kilns.
In accordance with one embodiment of the present invention there is provided an apparatus for delivering solid waste derived supplemental fuel into a preheater or precalciner cement kiln. Such kilns include a riser duct, a rotary vessel, and a shelf transition portion and, in operation, a kiln gas stream comprising combustion products flowing serially from the rotary vessel through the shelf transition portion and into the riser duct. The apparatus comprises a supplemental fuel delivery port in the riser duct spaced apart downstream, relative to kiln gas flow, from the transition shelf portion. Communicating with the fuel delivery port is a fuel delivery tube having a fuel inlet end external to the riser duct and a fuel outlet end spaced apart from the inlet end. A solid fuel feed mechanism is provided for advancing solid fuel elements through the fuel delivery tube and the fuel delivery port and into a supplemental fuel combustion region. The improvement of the invention comprises a controller for the fuel feed mechanism for adjusting the rate of advancement of the solid fuel element through the fuel delivery port and into contact with the kiln gas stream in a supplemental fuel combustion zone.
In one embodiment the apparatus further comprises a sensor in contact with the kiln gas stream and in communication electrically or telemetrically with a controller for providing signals indicative of combustion efficiency in the kiln. In one embodiment the sensor provides a signal indicative of the concentration of a gaseous component of the kiln gas stream. The controller can be programmed to increase the rate of advancement of the solid fuel element through the fuel delivery port in response to sensed increases in oxygen or nitrogen oxide concentration in the kiln gas stream, and to decrease the rate of advancement of the solid fuel element through the fuel delivery port in response to sensed increases in carbon monoxide or hydrocarbon concentration in the kiln gas stream.
The apparatus is preferably free of any supports of the fuel module in the riser duct. Optionally the apparatus can include a meal/dust delivery port in the riser duct independent of the fuel delivery port and a blower for dispersing meal or kiln dust into the kiln gas stream into or in the vicinity of the supplemental fuel combustion zone or region. Another aspect of the present invention is the optimization of the use of supplemental fuel in preheater/precalciner kilns with minimal impact on effluent kiln gas quality. The method comprises the steps of modifying a preheater or precalciner kiln to include the improved apparatus outlined in the aforestated embodiments and operating the kiln to minimize concentrations of carbon monoxide, hydrocarbons and nitrogen oxides in the effluent kiln gas stream.
Another embodiment of the present invention provides a method for controlling the feed rate of a fuel module into a combustion region, including the steps of determining the status of the combustion region, for example, gas composition or temperature, determining or programming the physical characteristics of the fuel module, and controlling the feed rate of the fuel module into the kiln as a function of the determined status of the combustion region and the fuel module characteristics. In one embodiment of the invention the determination of the status of the combustion region includes measuring the oxygen level in the combustion region or at a kiln gas downstream location where oxygen level of the combustion region can be determined indirectly. Alternatively, carbon monoxide levels or temperature levels can be measured to assess the status of the combustion region. downstream of the transition shelf portion. In one aspect of that method the size, shape, density of the fuel module can be measured and programmed into the controller. In one alternative embodiment the method can include the step of feeding a combustion control agent into the combustion region to affect the combustion. The feed rate of the combustion control agent can be monitored, adjusted, and coordinated with the feed rate of the fuel module.
In another embodiment of the invention there is provided a mechanism for feeding fuel modules into a combustion region. The mechanism includes a fuel module feeder, a sensor positioned in communication with the combustion region, and a control means for receiving signals from the sensor and data relevant to the physical characteristics of the fuel module and adjusting the rate at which the fuel module feeder feeds fuel modules into the combustion region responsive to said signals. The mechanism can optionally include a combustion control agent feeder positioned to deliver a combustion control agent into the combustion region to affect combustion of the fuel module in the combustion region. The combustion control agent feeder can be programmed to deliver the control agent to the combustion region at a continuous rate and signals indicative of that rate can be delivered to the fuel module feeder controller and used to set or reset the fuel module feed rate into the combustion region. Alternatively the feed rate of the combustion control agent can be adjusted in conjunction with controlling the rate at which the fuel module feeder delivers fuel modules into the combustion region.